Molybdenum disulfide electrical resistance devices



E. l.. KERN ETAL Sept. 2, 1969 MOLYBDENUM DISULFIDE ELECTRICALRESISTANCE DEVICES Filed Dec. 22, 1965 (um 14,1110) f/t//v//sysay :mwN\N\QQS N w m WM.N\

)lill 1 NVENTORS. dn/drol. Kern Or/'Jon J. 6'0//7 TTORNEY United StatesPatent Oliice 3,465,278 Patented Sept. 2, 1969 3,465,278 MOLYBDENUMDISULFIDE ELECTRICAL RESISTANCE DEVICES Edward L. Kern, Midland, andOrison J. Cain, Auburn, Mich., assignors to Dow Corning Corporation,Midland, Mich., a corporation of Michigan Filed Dec. 22, 1965, Ser. No.515,688

Int. Cl. H01c 7/04; H01b 1/10 U.S. Cl. 338-308 5 Claims The presentinvention relates to electrical resistance elements and moreparticularly to electrical resistance elements havingtemperature-controllable resistauces.

In various types of electrical circuits it is common practice to useyresistance elements which have temperature controllable resistancevalues. Such devices are used as sensors for measuring temperature andas thermistors in control circuits or the like, among otherapplications. In control circuits these devices may be either externallyheated or strictly self heated as desired.

While a number of materials exhibit a negative temperature-resistancecoeflicient over a range of temperature it is often desirable to havethe element variable over large temperature ranges or at very lowtemperature. By negative temperature-resistance coeflicient it is meantthat for each degree rise in temperature the electrical resistance ofthe element becomes lower. In materials having positivetemperature-resistance coefficients the converse is true. Most commonlyused electrical resistors exhibit increased resistivity withtemperature.

An object of the present invention is to provide an electrical resistorwhich exhibits a negative temperatureresistance coefficient over a widetemperature range including low temperatures, and which is economical toproduce. i

A further object is the provision of an improved negative temperaturecoefficient electrical resistance element having resistivities in thesemiconductor range. v

These and other objects of the present invention are achieved inaccordance with the invention by the provision of temperature sensitiveresistance elements formed of molybdenum disulfide. Molybdenum disulfide(MoS2) is found as a naturally occurring material in molybdenite. Inthis form it is a crystalline laminar material which delaminatcs readilyto a flake form and when ground has found substantial use as alubricant, besides being a primary raw material for productionofmolybdenum metal. It has been found, however, by the applicants thatthis material also lends itself to use inwspecial applicationthermistors, and other types of electrical resistance elements.

It has further been found that evaporated films of molybdenum disulfideexhibit a resistivity in the semiconductor range, also with a negativetemperature-resistance coefficient. The films can be tailored to givedifferent temperature-resistance coefficients as required for particularapplications.

Other objects and attendant advantages of the present invention willbecome obvious from the following detailed description when read inconnection with the accompanying drawings wherein:

FIG. 1 is a graph showing resistivity of bulk molybdenum disulfideversus the reciprocal of temperature, and

FIG. 2 is a graph showing resistivity of a thin film of molybdenumdisulfide versus temperature.

Bulk molybdenum disulfide has been found to have a negative temperaturecoeflicient of resistance over the range between K. and 500 K. It is alaminar material which delaminatcs readily. For use in electricalresistance elements flakes may be removed from large crystalline piecesand may `be easily cut to the shape required. For example, long narrowiilaments may be cut from a lamination of a desired thickness andelectrodes positioned at the ends thereof for connection in anelectrical circuit. As shown in FIG. 1 wherein resistivity in ohm-cm. isplotted on a logarithmic scale as a function of the reciprocal oftemperature in degrees Kelvin, the change in resistivity is typically 5%per degree over a range between about K. to about 330 K. (100G/Tapproximately equal to from 9 to 3). The curve shows only a portion ofthe over all resistivity-temperature characteristic for illustrativepurposes. This characteristic extends similarly both above and below thetemperature indicated. A device such as this makes an excellentthermistor in low temperature control circuits such as those used inlaboratories with liquid nitrogen. It has also been found that thin lmsevaporated from MoS2 and deposited on various substrates such as, forexample, glass plates, may be used with suitable electrodes affixedthereto to provide resistance elements having lower temperaturecoeicients of resistance than that of the bulk material. This isbelieved to be due to the effect of surface area and grain size incontrolling the mobility of charge carriers in the material. A plot ofresistivity in ohmcentimeters as a function of temperature in degreesKelvin is shown in FIG. 2 which illustrates a temperatureresistivitycoeicient of about 0.2% per degree. The thin film used in this plot wasdeposited on a glass plate and had a thickness of about 1 micron. Byincreasing thickness of the lmthe resistivity and temperature-resistancecoeflicient may both be increased to provide desired characteristics forparticular application. Additionally, other materials may be added tothe deposited M082 to change the electrical characteristics thereof.Also, the substrate may be either electrically conductive ornonconductive to provide desired characteristics. The films thus formedmay be used either as ordinary resistors or as compensating resistorsWhere small temperature compensation is needed.

Resistance elements of molybdenum disulfide may be deposited or formednot only by evaporation-deposition techniques, but by other methods aswell. Examples of other suitable methods include sintering in absence ofair and dry pressing with or without binders.

For temperature compensation the resistance elements formed inaccordance with the present invention may act either as self heated orexternally heated elements. Thus, higher current flow, causing higherheating will cause a decrease in resistance, altering current flow inthe circuit.

Obviously variations and modifications of the present invention willbecome obvious to those skilled in the art. It is to be understood,therefore, that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

That which is claimed is:

1. An electrical resistance element comprising a body consistingsubstantially of molybdenum disulde and means for supplying electricalcurrent to and conducting electrical current away from said body.

2. An electrical resistance element as defined in claim 1 wherein saidbody is cut from naturally occurring molybdenum disulde.

3. An electrical resistance element as dened in claim 1 wherein saidbody is a layer of molybdenum disulde 5 on a substrate.

4. An electrical resistance element as dened in claim 3 wherein saidlayer is a vacuum evaporated layer.

5. An electrical resistance element as defined in claim 4 wherein saidsubstrate is electrically nonconductive.

4 References Cited UNITED STATES PATENTS 2,609,470 9/1952 Quinn 252-518XR 3,111,567 11/1963 Stewart et al. 252-518 XR WILLIAM L. JARVIS,Primary Examiner U.S.Cl.X.R.

10 lll- 201, 106; 252-518g 338-308

1. AN ELECTRICAL RESISTANCE ELEMENT COMPRISING A BODY CONSISTINGSUBSTANTIALLY OF MOLYBDENUM DISULFIDE AND MEANS FOR SUPPLYING ELECTRICALCURRENT TO AND CONDUCTING ELECTRICAL CURRENT AWAY FROM SAID BODY.